1. Field of the Invention
The present invention relates generally to a seat belt arrangement for an automotive vehicle with an emergency lock function. More specifically, the invention relates to a so-called "mechanical buckle pre-tensioner" for providing higher tension for a seat belt in response to vehicular deceleration in a magnitude greater than a predetermined magnitude.
2. Description of the Background Art
In the modern automotive technologies, emergency lock seat belt arrangements have been developed in various constructions. The emergency lock seat belt arrangement generally operates to lock a seat belt for preventing extraction of the belt in response to extraordinary magnitude of acceleration which can be caused upon collision. Such arrangement is generally successful to protect a seat occupant from colliding onto vehicular structural components upon collision impact. On the other hand, the seat belt tends to tie the seat occupant onto the seat even at the normal state. Tying up the seat occupant may lead to a feeling on the seat occupant by loss of freedom and sometimes causes an uncomfortable ride feeling. In order to improve this, some of modern seat belt arrangements are provided with a tension reducing function to permit locking the seat belt at a position extracted in a length slightly more than that required for tying the seat occupant on the seat. This reduces tightness of the belt at a normal wearing condition and thus improves ride feeling.
For establishing an emergency lock function, an emergency lock seat belt retractor is normally employed in the emergency lock seat belt arrangement. The emergency lock seat belt retractor senses as inertia moment exerted on the vehicular body and establishes locking of the seat belt when the inertia moment becomes greater than a predetermined magnitude. Some of the emergency lock seat belt retractors employ a webbing sensor which senses extraction speed of the seat belt to establish locking at an extraordinary extraction speed of the belt. With such arrangement, locking is necessarily established after extraction of a certain length of the belt. Such extra length of the belt extracted will permit a certain magnitude of movement of the seat occupant. Particularly, when the seat belt arrangement employs the aforementioned tension reducer, the magnitude of movement of the seat occupant in response to the extraordinary deceleration becomes greater. As can be appreciated, upon vehicular collision, it is preferred to tie the seat occupant on the seat without permitting any motion. Therefore, the advanced technologies utilize such type of seat belt arrangement in combination with a pre-loader which forcingly retract the belt for tying up the seat occupant with an increased tension upon collision, or with an air bag system which forms an air bag cushion for preventing the seat occupant from colliding onto the vehicular structural component in front of the seat.
Such additional equipment necessarily cause increasing of the production cost. In order to avoid this, there have been proposed mechanical buckle pre-tensioners which draw a seat belt buckle in a tensioning direction to provide additional tension for the belt. Such type of the mechanical buckle pre-tensioners have been disclosed in Japanese Patent First (unexamined) Publications 61-241234, 62-289451 and so forth, for example. The mechanical buckle pre-tensioner employs a trigger mechanism for causing a pre-tensioning operation in response to collision shock. The trigger mechanism is relatively bulky. Since the trigger mechanism needs to be associated with the seat belt buckle, the trigger mechanism is preferably facilitated in the vicinity of the seat belt buckle. This causes interference with the vehicular structural components, such as a console box or so forth. Furthermore, since the modern vehicular seat employs a seat reclining mechanism for adjusting a seat back angular position, the construction around the seat becomes complicated to cause difficulty of installation and thus causes increasing of weight and cost.